This invention relates to a method and apparatus for aligning a media undergoing multiple passes in a media transporter which moves the media to specific desired locations and particularly to the improvement therein including apparatus for sensing a spatial pattern of densities in the media along a scanned strip, apparatus for saving the spatial pattern of densities during an initial pass of the media, and apparatus for comparing the saved spatial pattern of densities with the spatial patterns of densities sensed during subsequent passes, whereby the media can be positioned at desired positions exactly and repeatedly.
Electrostatic, electrophotographic, thermal, and pen printers and plotters of the type which record an image on a strip of recording media moving longitudinally across a transverse linear print head are well-known. Various techniques are known for using light reflected from or transmitted through the media to control such apparatus. For example, U.S. Pat. No. 4,178,095 to Champion et al. discloses employing reflective light-sensing to determine print quality on the media. U.S. Pat. No. 4,275,879 to Yamashita et al. discloses such a technique in connection with sheet feed detector. U.S. Pat. No. 4,670,647 to Hubbel III et al. discloses such a technique in connection with a selfcompensating document sensor which resists false reading in the presence of dust and the like. U.S. Pat. No. 4,468,113 to Motohashi et al. discloses a light transmissive media sensor system for determining paper thickness.
The media generally must be accurately aligned with the tracking sensor prior to operation. Color printers and plotters, particularly of the electrostatic type form color images by superimposing images of different colors on the recording media. Thus, each frame of the media must make multiple passes across the printer head in order to record the multiple images. For this purpose, after the first image is recorded, the direction of media travel across the head is temporarily reversed until the media returns to the starting position for recording of the second image. Then, the second image is recorded on the media as the media is again transported in the forward direction across the printer head. As can be appreciated, the accuracy with which the media is aligned with the printer head during the recording of each image directly affects image quality. If there is a slight error in this alignment, the image is misaligned in proportion to the error. Such misalignment reduces the accuracy of the resulting recorded image. If any misalignment occurs, false colors may also be recorded. Accordingly, each time the media travel direction is reversed and the media is returned to the starting location for the superposition of the next image, the alignment of the media with its original starting position is critical and directly affects image quality. Thus, such alignment presents a significant problem in such printers.
This problem is typically addressed in the prior art in the manner shown in FIG. 1 by the use of a pattern 10 recorded near the edge of the media 12 and a linear multi-pixel image sensor 14 (such as a charge coupled device (CCD)) connected to scan logic 16 which scans reflected light from the pattern 10 as the media 12 moves across the sensor 14 and develops a signal used by control logic of the printer or plotter to position the media 12. Typically, the recorded pattern 10 is a line with a series of rectangles along it. The CCD sensor 14 scans the media 12 and visually "reads" the pattern 10 to determine media position as the media 12 is returned to the starting position for the recording of the next image. The control logic of the printer or plotter processes the signal generated by the CCD sensor 14 to generate an error signal which is used to correct any misalignment of the media with respect to desired positions. The manner in which the signal generated by the CCD sensor 14 is used to correct the alignment of the media 12 with respect to the desired positions is well-known.
One significant problem with this prior art technique is that somehow the pattern must be printed on the media. If the track is pre-printed by the paper manufacturer, the cost of the media to the user increases significantly. If the printer prints the track (for example, during the printing), the cost of using the printer increases significantly. Moreover, the space occupied by the track reduces the amount of space available for recording an image on the media. If nothing else, the time spent printing the pattern is lost time which impacts the printer's overall throughput. Another problem with this prior art technique is that a high-cost linear image sensor (such as the CCD sensor 14) must be employed, which significantly increases the cost of the printer apparatus. Thus, there appears to be no known way of correcting for media position alignment errors in a media transporter without printing a pattern on the media and without installing a multi-pixel image sensor in the printer apparatus.
Accordingly, it is an object of the present invention to accurately correct media alignment errors in a media transporter (such as, for example, a multi-pass color printer) without requiring the printing of any pattern on the media and without requiring the use of a multi-pixel image sensor.
It is a further object of the invention to correct media alignment errors by exploiting only the characteristics of the media itself.
It is another object of the invention to correct media alignment errors by exploiting the spatial patterns of random media density fluctuations inherent in the media.
It is a still further object of the invention to correct media alignment errors by aligning the spatial pattern of random density fluctuations with respect to the media transporter.
It is yet another object of the invention to correct media alignment errors by recognizing spatial patterns of random media density fluctuations.
These and other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawings which accompany it.